Transthyretin (TTR), previously named prealbumin, is a soluble protein of 127 amino-acids (NCBI reference sequence: NP_000362.1) which is involved in thyroxin and retinol transport in the body. TTR is secreted in the blood by the liver and in the cerebrospinal fluid by the choroid plexus, and is also expressed in specific tissues like the pancreatic alpha cells or retinal epithelium. TTR synthesis starts at embryonic ages and continues during the whole life. It is present at high concentration in the plasma (3.6-7.2 μM) and CSF (0.04-0.4 μM) and typically forms under physiological conditions a soluble homotetramer of ˜55 kDa.
Under specific conditions which have been poorly elucidated and may include acidic pH, oxidative stress and local factors, the TTR protein adopts an alternative tridimensional conformation and becomes toxic.
The toxicity of misfolded TTR protein has been discovered by investigating a rare, autosomal dominant, neurodegenerative disorder named Familial Amyloid Polyneuropathy (FAP), which affects adult people in their midlife (Plantd-Bordeneuve et al., Lancet Neurol. 10 (2011), 1086-1097). FAP is characterized by progressive sensory, motor and autonomic impairments leading to death a decade after diagnosis. Nerve lesions are associated with the deposition of amorphous aggregates and amyloid fibrils made of TTR protein. The Val30Met substitution is the most frequent mutation causing FAP, especially in areas where the disease is endemic such as northern Portugal, but more than 100 different mutations have been already identified in the TTR gene; see Table IV below. The pathophysiological mechanism at play is identical for all the pathogenic mutations, in that the mutations alter the structural stability of TTR tetramer, promoting TTR misfolding and leading to the formation of toxic TTR species (Saraiva et al., Curr. Med. Chem. 19 (2012), 2304-2311).
TTR toxicity is also observed as a consequence of the Val122Ile mutation, which is found with high frequency (3-5%) in the African-American and West African populations. This mutation is associated with Familial Amyloid Cardiomyopathy (FAC), a condition where massive TTR accumulation in the myocardium leads to cardiac weaknesses and ultimately cardiac failure (Ruberg et al., Circulation. 126 (2012), 1286-1300).
Mutations in the TTR protein sequence are not a strict requirement for TTR toxicity, and the wild-type TTR protein is also prone to misfolding and formation of toxic aggregates. For example, Senile Systemic Amyloidosis (SSA) is characterized by cardiac weakness and the accumulation of wild-type TTR aggregates in the myocardium (Ikeda, Amyloid. 18 Suppl 1 (2011), 155-156; Dungu et al., Heart. 98 (2012), 1546-1554). Wild-type TTR deposits are also observed in multiple cases of ligament and tendon inflammations including carpal tunnel syndrome, rotator cuff tears and lumbar spinal stenosis (Sueyoshi et al., Hum. Pathol. 42 (2011), 1259-1264; Gioeva et al., Amyloid. 20 (2013), 1-6). Furthermore, TTR amyloidosis has been recently reported in the placenta of mothers suffering from preeclampsia (Kalkunte et al., Am. J. Pathol. 183 (2013) 1425-1436).
Treatments for diseases with TTR amyloidosis are limited and mainly invasive, wherein primarily the treatment is due to the symptoms. In the case of FAP, treatments rely on analgesics for the management of neuropathic pain, on liver transplantation to remove the main source for mutated TTR protein, and on treatment with Tafamidis. Tafamidis is a small molecule which binds to TTR tetramer and stabilizes its conformation. It acts against the dissociation of the TTR tetramer, the rate limiting step in the misfolding pathway leading to the formation of toxic TTR species. Tafamidis has been approved for the treatment of FAP in Europe but has not been approved in the USA, and its therapeutic efficacy is limited, in the best of cases, to slowing down disease progression. There is currently no treatment available targeting misfolded TTR protein.
In view of the above, novel therapeutic strategies are needed for an efficacious and safe therapy of diseases associated with TTR amyloidosis.
This technical problem is solved by the embodiments characterized in the claims and described further below and illustrated in the Examples and Figures.